This invention relates to the field of ultrasonic measurements, and more particularly, to a device for measuring the speed of sound in a gas using transit time of an ultrasonic pulse.
A unique physical characteristic of any gas is the speed of sound through that gas at specified pressure and temperature conditions. This characteristic, in combination with the temperature and pressure of the gas, may be used as a direct testing method for identifying individual pure gases and, further, may serve as one of several measurable physical parameters by which certain gas mixtures containing two or more known gas constituents may be quantitatively analyzed in terms of their concentrations. Application of speed of sound measurements in gases important to the natural gas industry is one in which the speed of sound, in combination with the gas temperature, pressure, and the amount of non-hydrocarbon diluent gases such as carbon dioxide and nitrogen, may be used to experimentally characterize the gas and infer the heating value energy content of the gas. In this application, the natural gas mixture normally contains methane as the primary hydrocarbon constituent (typically in the range of about 90-98 percent by volume concentration) together with variable small or trace amounts of heavier hydrocarbons (ethane through decane) plus carbon dioxide and nitrogen diluent gases at a total concentration in the range of a few percent by volume. Thus, with sensing techniques capable of indicating the gas temperature, pressure, speed of sound, carbon dioxide concentration, and nitrogen concentration, the heating value of the gas may be determined. See Morrow, T. P., Kelner, E. P., and Minachi, A. [20001]. xe2x80x9cDevelopment of a Low Cost Inferential Natural Gas Energy Flow Rate Prototype Retrofit Module,xe2x80x9d Final Technical Report, DOE Cooperative Agreement No. DE-FC21-96MC33033, Southwest Research Institute, San Antonio, Tex. In particular, this method of determining the energy content of natural gas does not require any additional information or measurements defining the hydrocarbon constituents or their concentrations. However, in order to determine the gas heating value to within an accuracy of xc2x10.1 percent as an acceptable value for gas tariff pricing and custody transfer, the measured parameters, including the speed of sound in the gas, must be determined to within approximately the same or a better degree of accuracy. Therefore, for this application a precision speed of sound sensor becomes an important component of an on-line natural gas energy flow meter. This sensor must operate reliably and accurately to provide the desired precision measurements under a wide range of field installations and ambient conditions and be capable of handling a full practical range of gas compositions common to the natural gas industry. The invention described herein refers to a methodology and apparatus for achieving speed of sound measurements with the desired high precision and reliability for applications associated with the natural gas industry and with users of natural gas. This method will also be recognized as having application in other speed of sound measurements, including tests in other gas compositions, in which high-accuracy results must be obtained.
One approach to measuring the speed of sound in gas involves measurement of the transit time of an ultrasonic pulse traveling over a known propagation distance in the gas. This technique typically employs one or more piezoelectric transducers to generate and detect sound waves in the frequency range of about 20 kHz to 1 MHz and higher. A particular technique known as a xe2x80x9cpulse echoxe2x80x9d technique uses a single transducer as both the transmitter and the receiver. The generated sound wave is reflected back to the source transducer from a target located at a known distance from the transducer, and is received by the same transducer. If the distance between the transducer and the reflecting target is D, and the measured two-way travel time is t, then the speed of sound is represented by:
Vgas=2D/t.xe2x80x83xe2x80x83(1)
This method is advantageous because it uses only one transducer. However, in applications requiring high-precision speed of sound measurements, the method has the disadvantage of introducing time delay errors associated with imperfectly defined and variable distance, D, and an imperfect ability to determine the exact time delay with respect to the time of initiation of the transmitted pulse and the time instant when the reflected sound wave is received at the transducer.
To reduce the time delay error, the pulse echo method may be modified to measure a time difference between two received signals. A transmitted wave is reflected from two different targets rather than a single target. The distance, d, between the two targets is known. Using this method, the speed of sound is represented by:
Vgas=2d/xcex94t,xe2x80x83xe2x80x83(2)
where xcex94t is the time difference between the two received signals.
With this two-reflector method, the two ultrasonic wave pulses returning to the transducer will be similar in amplitude and in waveform so that they may be accurately compared and their relative two-way travel time delay, xcex94t, measured. In particular, a method of cross correlation analysis may be used as the means for accurately comparing the two reflected pulses in a statistical sense and, in so doing, determine with good accuracy their relative time delay.
Regarding high-precision time-of-flight measurements, the disadvantage related to distance, D, in the single-reflector method is associated with the inability to accurately specify where, near the immediate face of the transmit-receive transducer, the sound wave is initiated in the gas and detected on reflection. More precisely, when the active face of the transducer and the face of the reflector do not have the same material, surface texture and finish, and acoustic impedance, the effective sound wave propagation path length may differ acoustically from the physically measured separation distance. Furthermore, the distance, D, will vary with temperature because of thermal expansion effects in the mounting structure holding the transducer and the reflector. Consequently, the single-reflector pulse-echo measuring arrangement must be calibrated as a function of temperature and appropriate temperature measurements of the mounting structure must be incorporated as part of the speed of sound measurement system.